In a typical telephone network, two types of echoes may be present: acoustical echo and electrical echo. Acoustical echo is susceptible to occur for example in a telecommunication network when a hands-free telephone terminal is used. The speech signal generated from the speaker in the terminal will be fed back toward the microphone of the terminal for example acoustically or mechanically. This signal will be transmitted back to the talker and therefore creates echo.
Electrical echo results from the presence of an impedance mismatch, such as a hybrid converter that is required to connect a full duplex four wire link to a two wire loop. Hybrids may be found, for example in telephones with two-wire analog interfaces or in network (PSTN) line cards. Due to an impedance mismatch in the hybrid converter, part of the received energy is reflected back to the transmitting port. Consequently, a talker hears his own delayed speech that, of course, is undesirable.
The traditional approach for reduction of echo in communication networks is to use echo suppressors. A typical echo suppress or acts like a switch that monitors the voice signals travelling in both directions. It detects which person is talking and blocks the signal ID travelling in the opposite direction. The drawback of such echo suppressors is that they tend to "chop" speech signals when the subscribers talk back and forth quickly due to the response time for monitoring the speech activities. Moreover, during double talk, i.e., when the subscribers talk simultaneously, the suppressor fails to control the echo.
One possibility to avoid the problems of echo suppressors is to provide circuitry or an algorithm that, instead of blocking speech signals in one direction in the communication link, cancels the echo by using an adaptive filter. In essence, an adaptive echo canceller synthesises the echo that is then subtracted from the composite signal (speech signal plus echo signal). A residual echo from this subtraction can still be at a noticeable level ii a practical echo cancelling system because the adaptive filter may not be able to model perfectly the true echo path as a result of time-varying echo path, insufficient filter length, non-linear effects, finite precision computations, etc. Consequently, a non-linear processor, such as a centre clipper (also called "switched loss device"), is often used with an echo canceller for further reduction of the residual echo. However, practical design of such devices may cause speech clipping or introduce clipping sounds that degrade the quality of the communication. Another difficulty arises as a result of the regulatory echo control targets established for existing echo cancellation devices. The targets are typically expressed as long-term averages, and may be overly conservative with quiet signals. A method is required that addresses the time varying speech levels found in practice.
Thus, there exists a need in the industry to provide an improved echo canceller, particularly well suited for use during a communication session involving at least one hands-free telephone terminal or a communication network comprising a hybrid converter.